In a mobile communication system, a communication terminal apparatus, when the power is turned on, searches for a cell to which it belongs (i.e. initial cell search), and searches for the cells as it moves across the cells (i.e. idle period cell search). Below, a cell search method in a mobile communication system of a W-CDMA/TDD scheme will be explained.
Every cell in a mobile communication system is assigned a scrambling code and a code group that corresponds to the scrambling code. As for the code group, there are four combinations of mid-ambles and scramble codes assigned such that they are not erroneously identified between neighboring cells.
Moreover, as shown in the control signal frame configuration diagram of FIG. 1, using predetermined slots in a frame (#0 and #8 in FIG. 1), by the timing toffset which is offset from the top of a slot by a predetermined time, a base station apparatus transmits the first synchronization channel (Primary Synchronization Code Channel: Cp) that is common to all cells, and the second synchronization channels (Secondary Synchronization Code Channel: Cs) that carry three codes to express a code group, simultaneously. As for the second synchronization channels, selecting 3 types from 17 types gives 4913=173 combinations, and out of these, the 32 least error detection types are used to express a code group.
Moreover, a second synchronization channel, Csj (j=1, 2, 3), is subjected to modulation that is 90°×n (n=0, 1, 2, 3) to the phase of the first synchronization channel Cp upon transmission. bj in FIG. 1 denotes the phase rotation amount in each second synchronization channel in relative to the phase of the first synchronization channel Cp.
As for the cells, these can be selected from Case 1, where synchronization channels are transmitted using one portion (the kth slot) of a frame (10 ms), and from Case 2, where synchronization channels are transmitted using two portions (the kth slot and the k+8th slot) of a frame (k is a whole number from 0 to 7).
When performing an initial cell search, for the first step, a communication terminal apparatus performs the correlation calculation of the first synchronization channel, and detects the timing giving the largest correlation value (hereinafter “peak timing”) as a slot timing.
Next, for the second step, the communication terminal apparatus performs the correlation calculation of 17 types of second synchronization channels and identifies the three types of second synchronization channels being transmitted from a base station apparatus. When identifying these second synchronization channels, the communication terminal apparatus uses 4-frame signals for Case 1, and 2-frame signal for Case 2. Then, the communication terminal apparatus identifies the code group assigned to its cell based on the phase rotation amount in four frames of the identified three types of second synchronization channels and the time offset value toffset of the synchronization channels from the top of a slot, and thus detects the timing of a frame top.
Finally, for the third step, the communication terminal apparatus performs the correlation calculation of the four types of mid-ambles belonging to the identified code group, and identifies the mid-amble and scrambling code assigned to its cell. Incidentally, to improve the characteristics of this mid-amble detection, the communication terminal apparatus performs wireless communication while synchronizing with the base station through AFC (Automatic Frequency Control).
In a mobile communication system of a W-CDMA/TDD scheme, thus, a communication terminal apparatus performs an initial cell search (scrambling code identification) in three steps.
Now, in the above cell search, a communication terminal apparatus is not frequency-synchronized with a base station, and so the oscillators in these apparatus have different oscillation frequencies, thus measuring one frame time differently.
However, since a conventional cell search method does not take the difference between frame times measured by a communication terminal and a base station apparatus into consideration, in the second step, correlation calculation is performed at a wrong timing that is off the peak timing.
For instance, if the one-frame time measured by a base station serves as a standard, and, in comparison thereto, the one-frame time measured by a communication terminal is shorter than that by the above base station by α[s], the communication terminal apparatus, in the second step, develops the difference from the peak timing by α[s] for every frame. If second synchronization channels are identified over four frames, a difference of maximum 4α[s] can result. In case this error develops large (greater than ½ chip time, for instance), the communication terminal apparatus becomes incapable of peak detection and detecting second synchronization channels.
Moreover, if the difference between the oscillation frequencies of the oscillators in a communication terminal apparatus and a base station apparatus grows large, the accuracy of synchronization detection in the early stages of AFC in the third step decreases, thereby making accurate mid-amble detection difficult.